The present invention relates generally to the drilling and exploration of a geological formation for the ultimate purpose of extracting hydrocarbons therefrom. More particularly, the invention relates to a system, apparatus, and method of conducting measurements from a wellbore or borehole penetrating the formation. The inventive system, apparatus, and method described herein are particularly suited for measuring a resistivity of a conductive formation fluid or mud in the borehole.
Formation fluids typically enter the borehole when the pressure in the formation exceeds the hydrostatic pressure. If the pressure differential is relatively large, the influx of formation fluids may be uncontrollable. This presents a danger of a well “blowout, which is of particular concern when the formation fluids include flammable hydrocarbons. Moreover, considerable expense is incurred in addressing the unsafe condition and restoring the well to a stable, and then operable condition. It is, therefore, desirable to monitor the borehole for such influx.
The resistivity of the formation fluid (i.e., oil, gas, and water) is typically different from that of the drilling fluid. Accordingly, measurement of borehole fluid resistivity during drilling allows for possible detection of a change in resistivity corresponding to an influx of formation fluids. Early detection provides time for corrective action to be taken. Corrective action entails mechanically sealing the well to prevent the escape of the formation fluids and/or increasing the weight of the drilling fluid by adding weighting material.
Borehole fluid resistivity measurements are also used in evaluating the geological formation adjacent the borehole. Typically, a geological formation that contains hydrocarbon has a higher resistivity than a formation that contains water. In a known method of detecting hydrocarbon-bearing formations, sensors adapted to measure resistivity of the formation are incorporated into the drilling apparatus so as to provide a continuous recording of formation resistivity during drilling. The sensors are designed to measure the resistivity of the formation; but are also affected by the resistivity of the fluid content of the borehole (“borehole fluids”). Accordingly, techniques are employed to correct formation resistivity measurements due to the influence of borehole fluid resistivity. Referred to as “borehole correction” methods, these methods require an estimate of the resistivity of the fluid entering the borehole. This estimate of borehole fluid resistivity may be obtained from knowledge of the chemical composition of the drilling fluids. Alternatively, this estimate may be obtained by measuring, at the surface, the resistivity of a sample of the drilling fluid taken from the borehole.
Borehole fluid resistivity sensors are included in some wireline logging tools. An example of such prior art borehole fluid resistivity sensors is described in U.S. Pat. No. 5,574,371, which has been assigned to the Assignee of the present application. The disclosure of the '371 patent serves as a good source of background information for the present invention. To facilitate the description of the present invention and to further highlight the present invention's contribution to the current art, the '371 patent is hereby incorporated by reference for all purposes and made a part of the present disclosure.
FIG. 1 provides one example of a prior art well logging apparatus used in measuring borehole fluid resistivity for borehole correction methods. Specifically, FIG. 1 provides an illustration of a prior art Auxiliary Measurement Sub (AMS sub) that is connected to a top of a well logging apparatus tool string, as originally disclosed in U.S. Pat. No. 5,157,605 to Chandler, et al. (which is also hereby incorporated by reference for all purposes and made a part of the present disclosure). The AMS sub includes a sub body 10 disposed in a wellbore 12 and defining an annular space 14 between the sub body 10 and a wall 16 of a formation penetrated by the wellbore 12. A multiple pin connector head 17 (typically, a 31 pin head) is connected to a top of the body 10 and another multiple pin connector head 19 is connected to a bottom of the body 10. The heads 17 and 19, which must withstand a high mud external pressure, are each very expensive to manufacture, costing about ten thousand dollars each. A conductive mud 18 is disposed within the annular space 14. The sub body 10 includes a recess 20 which is inwardly disposed relative to an outer wall of the sub body 10. A set of electrodes A1, M1, M2, and A2 are disposed within the recess 20. The electrode A1, called a current emitting electrode, is adapted to emit a current into the conductive mud 18, the current propagating through the mud 18 and being received by the electrode A2, called a current receiving electrode. The electrodes M1 and M2, called measurement electrodes, are disposed between the A1 electrode and the A2 electrode and measure a voltage potential drop which exists in a region 23 which is enclosed by a pair of equipotential lines 21, the region 23 including the conductive mud 18 in the wellbore 12 and formation penetrated by the wellbore 12. The voltage potential drop in region 23 of FIG. 1 is supposed to be representative of a resistivity (Rm) of only the conductive mud 18 in the annular space 14. However, a problem exists: the voltage potential drop in region 23 of FIG. 1 is actually representative of the resistivity of both the mud 18 and the formation penetrated by the wellbore 12. In order to avoid this problem, in FIG. 1, the AMS sub was purposely manufactured with the recess 20 so that the electrodes A1, M1, M2 and A2 could be placed within that recess 20. The reason for the recess 20 is as follows. When the electrodes were placed on the outer wall 10a of the sub body 10, the current being emitted from the current emitting electrode A1 would cross an interface (wall 16) which exists between the conductive mud 18 and the formation penetrated by the wellbore 12 thereby adversely affecting the accuracy of the measurement of the resistivity (Rm) of the conductive mud 18. When the electrodes A1, M1, M2, and A2 are placed in the recess 20, a much smaller quantity of the current, being emitted from the emitting electrode A1, is able to cross the interface 16 between the mud 18 and the formation. Consequently, that part of the voltage potential drop in region 23 resultant from the current flowing in the formation penetrated by the wellbore 12 was reduced; and, as a result, the adverse effect of the measurement of the resistivity (Rm) of the mud 18 in the wellbore 12 was reduced. However, as shown in FIG. 1, some current 22, called “crossing current” 22, from the emitting electrode A1 still crosses the interface 16 and flows in the formation penetrated by the wellbore. As a result, the voltage potential drop in region 23 of FIG. 1 enclosed by the pair of equipotential lines 21 and measured by the measurement electrodes M1 and M2 still includes both the potential drop of the conductive mud 18 and the potential drop in the formation penetrated by the wellbore 12. Therefore, when using the AMS sub of FIG. 1, the adverse effect of the crossing current 22 on the mud resistivity (Rm) measurement still exists and, as a result, the mud resistivity measurement is not as accurate as desired.
The specification of the '371 patent describes a prior art measurement probe 26 for measuring borehole fluid resistivity that is disposed adjacent the bottom of a wireline tool string, as shown in FIG. 2. The measurement probe 26 includes a current emitting electrode A0, and measurement electrodes M1, M2, and current receiving electrode A1. In operation, current I0 emitted by the emitting electrode, A0, is propagated from the electrode array in a downhole direction 24c that is approximately parallel to the longitudinal axis 24b of the probe 26 (and the wellbore). The current I0 is therefore directed into the wellbore space below the wireline tool as opposed to the direction of the formation. In this way, some of the problems of the above-mentioned prior art, as disclosed in the '605 patent and/or illustrated by FIG. 1, are addressed. Specifically, the “crossing currents” and thus the influence of the formation on the borehole fluid resistivity measurements are reduced or eliminated.